There are basically two known different types of domestic electric radiator. Firstly, electric convectors, for which the ambient air to be heated is directly in contact with an electrical heating resistance. These electric convectors are in widespread use but have the drawback of generating a significant movement of the ambient air because of the temperature gradient created, causing a feeling of discomfort for the occupants of the room under consideration. This problem is partially resolved by another type of so-called radiant radiator operating by radiation.
Also known are radiators with a heat-transfer fluid, wherein said fluid, generally oil, is heated by means of an electric heating element and is conveyed in a heater, in which the heat is transferred to the ambient air by natural convection. Given the presence of a heater with a relatively large heat transfer surface, the temperature gradient with the ambient air is reduced so that the movements of air by natural convection in the room concerned are limited.
Among these radiators with a heat-transfer fluid may be distinguished firstly radiators in which the fluid operates in single-phase. In the case in point, said fluid remains in the liquid state. In this case, the heat-transfer fluid is heated in contact with an electric heating element, thins out and rises inside the heater. As it moves gradually upward, the heat-transfer fluid gives up some of the heat to the ambient air through the wall of the heater, and consequently cools down. Since the fluid so cooled becomes denser, and therefore heavier, it drops back down by gravity into the lower part of the radiator. To operate this type of radiator properly, it therefore proves necessary to have a minimum temperature difference between the rising (hot) fluid and the dropping (cold) fluid that is directly dependent on the fluid pressure drops generated by its circulation. This type of radiator thus sees a non homogeneous distribution of the temperature of the heater wall, that affects the efficiency of the radiator. Moreover, this type of operation may induce hot points on the surface of the appliance that are dangerous and additionally incompatible with decreed safety standards.
To overcome these drawbacks, a radiator with a heat-transfer fluid has been proposed, for example in the documents GB-A-2 099 980 and WO-A-02/50479 that operates in two-phase form, and particularly liquid/vapour. Said radiator operates as follows: The heat-transfer fluid in the liquid state lies through gravity in the lower part of the radiator passed through by a heating element, constituted by a temperature raised fluid, and passing through the base of said radiator in a leak-tight manner.
Under the effect of the heat, the heat-transfer fluid is vaporized, said vapour then rising in the internal structure of the radiator, and particularly in a heater, in which a transfer of heat occurs. Consequently, because of the temperature of the walls of said heater, lower than that of the vapour, the latter condenses. The condensate so formed comes in liquid form, and returns by gravity alone to the lower part of the radiator.
Because of the heat transfer method, in the case in point by phase change, bringing the latent condensation heat directly into play, an almost homogeneous heater wall temperature is thus ensured, thereby constituting a very clear improvement relative to radiators with a heat-transfer fluid that operate in single-phase. Indeed, this transfer temperature is very close to the saturating vapour temperature of the heat-transfer fluid because of a heat exchange factor that is appreciably higher in condensation than by natural convection on the outside, i.e. on the ambient air side. A substantial gain is thus obtained in respect of the variation in air temperature.
However, the hot source raising the temperature of the heat-transfer fluid proves tricky to control, both in time and in space. Moreover, it can be seen that if the vaporization rate of the heat-transfer fluid is too high, the vapour so generated produces drops of heat-transfer fluid, disturbing the proper operation of the radiator.
Moreover, with two-phase radiators of this kind, a noise problem is encountered too when they start up. This noise is caused by pressure waves when the vapour bubbles collapse in the sub-cooled liquid. Depending on the fluid used and the quantity of liquid fluid introduced into the body of the radiator, this noise phenomenon is more or less significant. In fact, this noise pollution may prove bothersome, or even totally unacceptable for some uses, such as in particular hospital wards, convalescent homes, retirement homes, or even just bedrooms.
Furthermore, when the heating element is directly in contact with the heat-transfer fluid for the heating thereof, as is the case for example in the document WO-A-02/50479, it may be damaged when the volume of liquid is too small. Indeed, the vapour phase, in which the heating element is for the most part, if not entirely, soaked, is not sufficient to absorb the energy of the heating element which may then overheat.
Additionally, using a radiator with a heat-transfer fluid operating in two-phase form means that it has to be mechanically strong because of the pressure exerted on the walls by the vapour which is under pressure given the enclosed space in which it is trapped. This generally means that the radiator has to be oversized and/or thick walls used thereby taking up space and involving extra cost.
A two-phase fluid has also been proposed in the document EP 0 281 401, wherein said fluid is constituted by two different heat-transfer liquids, in the case in point glycol ethylene and water.